Because carburetors supplying fuel to general purpose engines are small, they mostly have simplified fuel systems. Well-known carburetors include fixed venturi carburetors using a single fuel system in which a nozzle orifice is opened in the narrowest portion of venturi tube, as described in Japanese Patent Application No. 46-10565, and variable venturi carburetors using a single fuel system in which a nozzle orifice is opened in a variable venturi tube of a slide throttle valve type disclosed in Utility Model Application No. 49-17682.
The advantage of using a single fuel system is that a fuel flow rate smoothly transitions from a low-speed operation range to an intermediate or high-speed operation range. Furthermore, the advantage of adding a mechanism for mechanically adjusting the fuel flow rate in response to the open-close operation of a throttle valve to such a system is that the air/fuel ratio is maintained within a preset range corresponding to the fuel flow rate and the air flow rate. Moreover, the introduction of bleed air is advantageous because it optimizes the fuel flow rate and improves formation of fine droplets of fuel discharged into the intake channel.
A mechanism for adjusting the fuel flow rate includes inserting a metering needle into a fuel nozzle adjusting the effective surface area and also represents the conventional technology. Moreover, in such a structure, bleed air is introduced between the main jet of a fuel passage and a nozzle orifice, and the flow rate of bleed air introduced into the fuel passage is determined by the difference in pressure between the bleed air inlet opening and nozzle orifice.
However, when the intake negative pressure generated during idling of general purpose engines was continuously measured, it was found that the intake negative pressure was not constant and was changing cyclically. Negative pressure acting in the nozzle orifice changes under the effect of these changes in the intake negative pressure. As a result, the difference in pressure between the nozzle orifice and bleed air inlet opening and the difference in pressure between the nozzle orifice and constant-fuel chamber also change, disturbing the air/fuel ratio in the air-fuel mixture supplied to the engine and, thus, destabilizing idling. Destabilization of idling causes cyclic degradation because it increases variations of the intake negative pressure and further destabilizes idling.
In engines for general applications, the quantity of discharge gases is small and the required fuel flow rate is low. Therefore, the effect produced by changes in the fuel flow rate during idling cannot be ignored.